Field of the Invention
The present invention relates to an optical writing device and an image forming apparatus, and particularly to a technology for preventing non-uniformity of light intensity of an optical writing device which uses an organic LED.
Description of the Related Art
In recent years, an optical writing device (PH: Print Head) including organic LEDs (OLEDs: Organic Light Emitting Diodes) has been proposed as a component equipped on an image forming apparatus with an aim of miniaturization and cost reduction of the image forming apparatus. OLEDs are disposed on a TFT (Thin Film Transistor) substrate and arranged in lines in a horizontal scanning direction, and electrically connected in parallel via power source wiring similarly arranged in the horizontal scanning direction (FIG. 10).
An OLED is called an organic EL (Organic Electro-Luminescence) element as well, and provided as a current driven light emitting element. When driving current is supplied to an OLED via power source wiring, a voltage drop occurs along the power source wiring due to wiring resistance.
On the other hand, a driving circuit which generates driving current for an OLED is provided for each OLED at a position adjacent to the corresponding OLED, and generates driving current in reference to an electric potential at a junction point between the driving circuit and the power source wiring. Accordingly, the voltage drop at the power source wiring produces a drop of the reference potential, in which condition the amount of driving current to be supplied to the OLED is variable. In this case, the light emission luminance becomes variable, and non-uniformity of images may be caused (FIGS. 11A and 11B).
For overcoming this problem, reduction of impedance of power source wiring has been proposed, for example (JP 2005-144685 A, JP 2005-144686 A, JP 2005-144687 A, and JP 2010-076184 A). According to this method, the voltage drop produced by driving current is avoidable, wherefore the non-uniformity of images can decrease.
According to the foregoing conventional technology, power source wiring is further formed on sealing glass provided for sealing the TFT substrate, and the power source wiring on the TFT substrate and the power source wiring on the sealing glass are electrically connected by connecting parts at respective power supply points of the driving circuit, for the purpose of reduction of impedance of the power source wiring. In this case, there is a problem that the unit cost rises. Moreover, the auxiliary power source wiring thus formed is thin-film wiring, wherefore reduction of impedance of the power source wiring is limited.
According to another conventional technology, one line cycle is divided into a sample period and a hold period. During the sample period, OLEDs are turned off, and a luminance signal output from a DAC (Digital to Analogue Converter) circuit is temporarily held in a sample hold circuit (hereinafter referred to as “S/H circuit”) provided for each OLED. During the hold period, driving current in correspondence with the luminance signal held in the S/H circuit is supplied to each OLED to allow light emission therefrom.
According to this structure, no driving current flows during the sample period, in which condition no voltage drop occurs. Accordingly, the luminance signal is appropriately sampled, wherefore non-uniformity of luminance caused by a voltage drop is avoidable.
However, while a method so-called rolling driving turns off an OLED only when the luminance signal is input to the corresponding S/H circuit, this conventional technology turns off OLEDs throughout the sample period in which luminance signals are sequentially input to a number of S/H circuits, and only turns on the OLEDs during the hold period. In this case, light emission duty corresponding to a proportion of a light emission period in a horizontal scanning period (Hsync) lowers, wherefore the light emission period becomes short.
When the light emission amount from the OLEDs is raised by increasing the amount of driving current supplied to the OLEDs so as to obtain sufficient exposure during the short light emission period, the life of the OLEDs may decrease.